1. Field of the Invention
The present invention relates to a control device that controls an induction motor without using a speed sensor and a control method of the control device.
2. Description of Related Art
In general slip frequency type speed-sensorless control, a primary frequency is calculated by adding a slip frequency to estimated motor speed, and a magnetic flux phase is calculated by integrating the primary frequency. The slip frequency is calculated on the basis of set secondary resistance. A value measured using an auto-tuning method, a value obtained by a combined test, or a motor design value is used as the value of the secondary resistance.
When a motor is driven, the value of the secondary resistance varies with a change in temperature. Thus, the slip frequency cannot be correctly calculated. For example, “Rotor Resistance Adaptation for Sensorless Vector Controlled Induction Machines”, H. Kubota et al. (T. IEE Japan, Vol. 117-D, No. 8, 1997, pp. 940-945) proposes, as a solution to this problem, obtaining the value of secondary resistance by applying high frequency components to d-axis current while driving a motor and separating a speed error from a secondary speed error.
Moreover, a method in which a thermistor is built into a motor, primary resistance is directly measured using the thermistor, and the value of secondary resistance varying with temperature is obtained, assuming that the rate at which the primary resistance varies with temperature is equal to the rate at which the secondary resistance varies with temperature, is also known.
Moreover, for example, Japanese Unexamined Patent Application Publication No. 2002-125400 proposes, as a method for identifying primary resistance without using a thermistor, estimating primary resistance from two-phase voltage of direct-current components generated upon receipt of a command value, two-phase current converted from motor current, and the estimates of magnetic flux and speed.
Moreover, Japanese Unexamined Patent Application Publication No. 2002-253000 proposes defining an hv-axis rotating coordinate system that is delayed from a dq-axis rotating coordinate system by a phase angle θhv equal to a primary current phase angle θIdq, calculating an h-axis induced voltage error dEh and a v-axis induced voltage error dEv, and correcting the set value of primary resistance so that the calculated v-axis induced voltage error dEv converges to zero.